The invention relates to internal combustion engine supercharged by at least one turbocompressor comprising at least one compressor whose output is connected to an intake manifold of said engine drivably connected to a turbine which receives the exhaust gases of the engine. The term "internal combustion engine" should be construed as designating any engine whose combustion takes place in a variable volume chamber which may be pressure ignited or spark ignited, reciprocating or rotary.
The invention is suitable for use each time the engine supplies, at its rated operating point, exhaust gases whose energy is sufficient for the compressor to increase the pressure of the air drawn in by the engine. A particularly important, although not exclusive, use is in the field of propulsion engines which must provide a sufficient torque at low running speed and must be capable of providing a very variable power at any rotational speed, (assuming that engine ignition does not require a minimum air intake pressure): this case is that of petrol engines and Diesel engines with a high compression ratio used for propelling motor vehicles.
The problems of matching which arise when associating a turbocompressor, i.e. a continuous flow machine whose delivery pressure is directly related to the airflow which passes through it by a so-called "pressure-flow characteristic field", (which depends solely on the geometry of the compressor and of the turbine) and an engine, a positive displacement machine with discontinuous flow, have been known for a long time and have not up to the present received a fully satisfactory solution. Before describing the invention, it may be useful to recall them while emphasizing those which are particularly acute in the case of propulsion engines and, more especially, motor vehicle engines.
As mentioned above, the relationship between the delivery pressure of the compressor and the airflow which passes therethrough is different from that corresponding to the operation of a reciprocating engine, which absorbs a volume of air flow substantially proportional to its running speed, at a maximum pressure which results from mechanical or physico-chemical considerations. If the turbocompressor is dimensioned so that the supercharging pressure which it supplies never exceeds the maximum acceptable value for the engine, the action of the supercharging turbocompressor is negligible as long as the load on the engine does not approximate its maximum load and the torque of the engine at low speed is very low, substantially equal to that of a non-supercharged engine having the same chamber size. To overcome this problem, additional adjustment of the turbocompressor must be provided. The solution most widely used at the present time consists in using a turbocompressor whose characteristic in the pressure-flow field of the compressor is such that the maximum acceptable supercharging pressure for the engine is reached under partial load and in providing the engine with a "waste gate", which opens the exhaust manifold to the atmosphere. This solution has the advantage of being very simple. But, on the other hand, it presents numerous drawbacks: when the reciprocating engine is running at high speed, it leads to a high value of the exhaust pressure, so to a counter scavenging, to a power drop and to an increase in the consumption of the engine per unit power. Furthermore, it corresponds, under rated operating conditions of the supercharged engine, to operation of the turbocompressor at a point of operation in the pressure-flow field of the compressor at which its efficiency is low.
It has also been proposed to use a turbocompressor whose turbine comprises two cavities (French 2 465 069). This solution provides satisfactory operation at high running speeds. On the other hand, at low running speeds, the second cavity is not fed with exhaust gases. It then operates as a centrifugal compressor of poor efficiency and detrimentably affects the power and the efficiency of the engine in the range in which it is precisely most used, approximately between 20 and 60% of its rated power.
It has also been proposed to provide the input of the turbine with an adjusting valve (FR-A-2 393 999) or to give a variable geometry to the turbine casing (FR-A-2 485 634). But very little influence is exercised on the momentum of the gas flow which defines the pressure-flow rate law of the turbocompressor, so that the improvement obtained is negligible. Moreover, these two solutions involve the presence of moving mechanisms in the very hot exhaust gases, whence a risk of a short useful life.
It is an object of the invention to provide an improved engine supercharged by at least one turbocompressor; it is a more specific object to reduce to a very large extent the above-mentioned disadvantages and, in particular, to maintain a high degree of efficiency of the turbocompressor at all running speeds at which the engine supplies power.
To this end, the invention proposes an internal combustion engine supercharged by at least one turbocompressor comprising at least one compressor whose output is connected to the intake manifold of the engine, driven by a turbine which is driven by the exhaust gases of the engine, characterized in that it comprises a take-off duct bringing a fraction of the airflow delivered by the compressor to the exhaust gases and in that the turbine comprises distributor vanes with variable setting.
The supply of air brought to the exhaust gases reduces temperature of the gases admitted into the turbine and consequently, allows reliable operation of the mechanism for orientating the distributor vanes. The compressor(s) will be generally dimensioned as a function of the engine so that the dilution rate (ratio of the airflow taken off to the airflow passing through the engine) is less than 3. As soon as a value of 1 is reached, the maximum temperature of the exhaust gases is reduced to a value of about 550.degree. C., perfectly compatible with the presence of movable adaptation members. A non return device may be provided in the duct so as to prevent the exhaust gases from returning to the intake.
Under all the conditions in which the engine exerts a driving torque, the pressure at the delivery of the compressor is greater than the pressure at the intake side of the turbine. This fact may be advantageously used by injecting the compressed air coming from the compressor and passing through the off-take duct by means of an ejector which opens into the exhaust pipe of the engine, upstream of the turbine.
The range of movement of the vanes will depend to a large extent on the desired effect. A setting up to about -10.degree. may be envisaged, which allows the engine to be undercharged, and as far as a setting of +70.degree. at tick-over. But, in practice, a movement range having an amplitude of 40.degree., going for example from 20.degree. to 60.degree., will be perfectly acceptable when the rotational speed of the engine under load varies from 2000 rpm to 5500 rpm. This reduced range of variation of the setting of the vanes is sufficient for the angle of incidence of the gases on the rotor vane grid to vary by only a few degrees and for the angle at the output not to exceed .+-.10.degree. for constant enthalpy in the compressor and for a constant dilution ratio.
The presence of the by-pass airflow allows favorable results to be obtained which are added to those already mentioned: this take-off flow, since it is continuous, contributes to attenuating the speed and pressure pulsations of the exhaust flow of the engine, and so improves the efficiency of the turbine. The dilution plays a role of silencer which is added to that of the turbine. The reduction of the maximum temperature at the input of the turbine allows the clearances between rotor and stator to be reduced and so the efficiency of the turbine to be further improved. It reduces the angular rotational speed of the turbocompressor by .sqroot.1+.lambda.(.lambda. being the dilution ratio) with a favorable effect on the linear speed of the bearings and moreover on the efficiencies by Reynolds effect. Finally, by reducing the counter pressure in the exhaust, a sweep is provided ensuring better filling of the cylinders.
The invention will be better understood from reading the following description of particular embodiments of the invention given by way of non limiting examples.